1. Field of the Invention
The present invention relates to a ballpoint pen tip that can prevent backflow of ink, to a ballpoint pen utilizing the ballpoint pen tip and to a method of manufacturing the ballpoint pen tip.
2. Description of the Prior Art
As writing instruments, ballpoint pens such as oil-based ballpoint pens or gel ink ballpoint pens have been widely used. Such a ballpoint pen generally has a structure that a ballpoint pen tip including a writing ball is provided in a front end thereof, and that an ink-containing portion that contains ink is provided in the rear of the ballpoint pen tip.
Further, the front end of the ballpoint pen tip is generally tapered inward to form a caulking portion in order to prevent detachment of the writing ball from the ballpoint pen tip. Between the caulking portion and the writing ball of the ballpoint pen tip, a predetermined clearance is so formed that the ink passes freely therethrough. Not only the ink but also air can pass through the clearance.
Thus, during writing with a pen point faced upward, the air may enter from between the writing ball and the caulking portion. If the entry of the air is set aside, the ink may flow backward to leak from the rear end of the ink-containing portion. This may cause inconvenience such that writing with the ballpoint pen becomes impossible, or that the leaking ink dirties clothes.
For a gel ink ballpoint pen using gel ink as writing ink, attachment and detachment of a protection cap over a pen point may axially impact a rear end of the ballpoint pen. The impact may cause minute backflow of the ink in the ballpoint pen tip to prevent writing at a writing start or create thin spots in drawn lines, so-called impact thin spots.
In a general ballpoint pen, an opening area of a clearance between a writing ball and a caulking portion becomes larger for a larger diameter of the writing ball, and thus air tends to enter the ballpoint pen tip during writing with the pen faced upward. For example, if a ballpoint pen having a writing ball with a diameter of 0.7 mm or more is used to write with the pen faced upward, air tends to enter the ballpoint pen tip to cause backflow of ink. Particularly, for a gel ink ballpoint pen having a writing ball with a diameter of 0.7 mm or more, air tends to enter the ballpoint pen tip to cause backflow of ink, thus noticeably causing inconvenience such as impact thin spots.
In order to eliminate such inconvenience, a ballpoint pen having a check valve mechanism that prevents backflow of ink is used. Known check valve mechanisms have configurations as described below.
Specifically, a check valve mechanism is known in which a joint, which connects a ballpoint pen tip and an ink-containing tube that contains ink and is mounted therebetween, is provided, a valve-housing chamber that houses a spherical or substantially cylindrical valve is provided in the joint, and the valve is movably placed in the valve-housing chamber (c.f. JP 10-236063 A1). According to the check valve mechanism, when the ballpoint pen is faced upward, the valve closes an ink-passing hole bored in an inner surface of the valve-housing chamber to prevent backflow of the ink.
Another check valve mechanism is known in which a valve-housing chamber that houses a spherical valve is provided in a rear end of a ballpoint pen tip, and with the valve movably housed in the valve-housing chamber, a rear end opening of the ballpoint pen tip is caulked to prevent detachment of the valve (c.f. JP Utility Model Laid-Open No. 57-4373). The check valve mechanism having such a configuration can prevent backflow of the ink.